Export collection and replace field with field from another collection (aggregate?) - mongodb

Question

Using MongoChef GUI but fine in command line.
I have a collection with a structure as thus:
Votes
{
"_id" : "5qgfddRubJ32pS48B",
"createdBy" : "HdKRfwzGriMMZgSQu",
"fellowId" : "yCaqt5nT3LQCBLj8j",
}
I need to first look up the user in a users collection using the createdBy field to see if they are verified
Users
{
"_id": "HdKRfwzGriMMZgSQu",
"emails" : [
{
"address" : "someuser#example.com",
"verified" : true
}
]
}
and additionally, get some more information from a third collection from fellowId
Fellows
{
"_id" : "yCaqt5nT3LQCBLj8j",
"title" : "Fellow Title"
}
And have them all export as one csv or json file. How can I achieve this as a mongo query/export?
The desired output would be, for example:
{
"_id" : "yCaqt5nT3LQCBLj8j",
"fellowTitle": "Fellow Title"
"isVerified" : true
}

Answer 1

You can perform an aggregate with 2 $lookup to join both collections :
1 $lookup to join users
1 $unwind to remove users array
1 $unwind to remove user email array (as we have to check verify)
1 $sort to sort with user.emails.verified
1 $group to actually pick only the first entry (verified or not)
1 $lookup to join fellows
1 $unwind to remove fellows array
1 $project to format whatever format you want at the end
1 $out to export to a new collection
Query is :
db.votes.aggregate([{
$lookup: {
from: "users",
localField: "createdBy",
foreignField: "_id",
as: "user"
}
}, {
$unwind: "$user"
}, {
$unwind: "$user.emails"
}, {
$sort: { "user.emails.verified": -1 }
}, {
$group: {
_id: "$_id",
createdBy: { $first: "$createdBy" },
fellowId: { $first: "$fellowId" },
user: { $first: "$user" }
}
}, {
$lookup: {
from: "fellows",
localField: "fellowId",
foreignField: "_id",
as: "fellow"
}
}, {
$unwind: "$fellow"
}, {
$project: {
"_id": 1,
"fellowTitle": "$fellow._id",
"isVerified": "$user.emails.verified"
}
}, {
$out: "results"
}])
Then export with :
mongoexport - d testDB - c results > results.json